1. Field of the Invention
The present invention relates to a magnetron driving circuit, and more particularly to a magnetron driving circuit capable of preventing the production of a reverse surge voltage during a supply of direct current power.
2. Description of the Related Art
Generally, a microwave oven is a device for cooking food by using microwaves, and has a high voltage transformer (hereinafter called HVT), and a magnetron MGT. The HVT steps up the normal voltage into the higher voltage, and the magnetron MGT is driven by the higher voltage to generate the microwaves of a certain frequency.
Meanwhile, such a microwave oven is designed to be driven by alternating current (hereinafter called AC), and can not be used in the places such as the outdoors, vehicles such as ship, airplane, etc., where the AC is not available. In order to solve such a shortcoming of the microwave oven, an inverter is used to convert the direct current (hereinafter called DC) into the AC for using the microwave oven in the place where the AC is not available.
The AC generated by the inverter is stepped up by the HVT to drive the magnetron MGT. Here, when the DC voltage is converted into the AC power and is outputted by the inverter, there occurs a reverse surge voltage induced at a primary part of the HVT from a secondary part of the HVT, which generates a spark at the inverter. For example, before the high voltage capacitor (hereinafter called HVC) at the secondary part of the HVT is charged, the secondary side circuit forms the short circuit and the reverse surge voltage occurs at the primary coil, resulting in a spark at the inverter. Further, after the HVC is charged, the energy of the secondary coil is reversely induced to the primary coil every half-period, again resulting in the spark at the inverter due to the energy reversely induced.
Hereinafter, the construction, operation, and problems of the inverter driven by the DC power and a magnetron driving part connected with the inverter will be briefly described as a related art.
There are various types of invertors such as the inverter using a relay, and the inverter using semiconductor devices, etc. The same applicant has disclosed a non-directional frequency generator (hereinafter called NDFG), which is an improved version of the inverter, in the Korean Patent Application, and here, the construction, operation, and the shortcomings of the NDFG and the magnetron driving section connected thereto will be described.
The NDFG converts the DC power into the AC power source by using rotatable AC converting means, and is disclosed in the Korean Patent Applications Nos. 98-18589 (filed May 22, 1998), and 98-21117 (filed Jun. 8, 1998) which have not been opened to the public yet.
FIG. 1 is a circuit diagram for showing the NDFG driven by the DC power and the magnetron driving part connected thereto according to the related art of the present invention. Referring to FIG. 1, the NDFG 100 includes a motor 110 driven by the DC for generating rotational force, a commutator 130 rotated by the motor 110, and a plurality of brushes such as first, second, third, and fourth brushes 121-124 as shown in FIG. 1, which are in contact with the outer circumference of the commutator 130. The commutator 130 includes a conductive part which is divided into at least two parts 132a and 132b as shown in FIG. 1, but into an even number of parts. The conductive parts 132a and 132b have an insulating part 133 of a certain width formed therebetween. The conductive parts 132a and 132b are in simultaneous contact with at least two neighboring brushes 121-124. The DC is applied to the input sides of the first to fourth brushes 121-124, while the output sides of the first to fourth brushes 121-124 are connected with a high voltage transformer (hereinafter called HVT). The first and second relays RY.sub.1 and RY.sub.2 switch on/off the operation of the NDFG 100.
The operation of the NDFG 100 is as follows: The first and second relays RY.sub.1 and RY.sub.2 are in on-state, and the commutator 130 is rotated by the DC. Accordingly, the brushes 121-124 in contact with the commutator 130 come in contact with the conductive part 132a, the insulating part 133, the conductive part 132b, and the insulating part 133 which are formed on the outer circumference of the commutator 130, sequentially.
More specifically, as the first brush 121 on the upper side of the commutator 130 comes in contact with the conductive part 132a, the electric current from the positive (+) terminal of the DC power source is inputted into the first brush 121, and flows through the conductive part 132a of the commutator 130 and the fourth brush 124, and to the upper portion of the primary coil 202 of the HVT downwardly to the lower portion of the primary coil 202 of the HVT. Then, the electric current is inputted into the second brush 122, and circulates through the conductive part 132b, the third brush 123, and to the negative (-) terminal of the DC power source.
Next, as the commutator 130 is further rotated and as the first brush 121 accordingly comes in contact with the insulating part 133, the electric current does not flow through the commutator 130.
Then as the commutator 130 is further rotated to 90.degree., the electric current from the positive (+) terminal of the DC power source is inputted into the first brush 121, flows through the conductive part 132b of the commutator 130 and the second brush 122, reverses its direction, and flows from the lower portion of the primary coil 202 of the HVT to the upper portion of the primary coil 202 of the HVT. Then, the electric current is inputted into the fourth brush 124, flows through the conductive part 132a, and the third brush 123, and then circulates to the negative (-) terminal of the DC power source.
By the constant rotation of the commutator 130 of the NDFG, the AC is generated at the primary coil 202 of the HVT in a manner as described above, then the AC is transmitted to a secondary coil of the HVT through the primary coil 202 thereof. Then, the HVT converts the normal voltage into a higher voltage, and the magnetron MGT is driven by the higher voltage converted by the HVT.
When the magnetron is driven, there occurs a problem that the secondary circuit forms a short circuit until the high voltage capacitor HVC of the secondary part of the HVT is charged. That is, when the AC induced from the NDFG 100 is applied to the HVT, the high voltage capacitor HVC connected to the secondary coil of the HVT is shorted instantaneously, and thus a reverse surge voltage occurs in the primary coil. Nearly infinite inrush current due to the reverse surge voltage generates a spark between the brushes and the commutator of the NDFG 100.
Furthermore, even after the high voltage capacitor HVC is normally charged, there occurs another problem that the electric energy of the secondary coil is induced reversely to the primary coil every half period. The reversely induced electric energy generates a spark between the brushes and the commutator of the NDFG 100.
Meanwhile, the problems do not only occur between the magnetron driving part and the NDFG driven by the DC, rather they occur between the magnetron driving part and the inverter in a broad sense for inverting the DC to the AC, including the NDFG.